You can't really mix your MySQL "show databases" concept with Oracle as the underlying philosophy is different. In MySQL a single engine is used to maintain all databases, but Oracle has a separate "engine" for each database - i.e. nothing is shared unless you have a clustered db - so the databases are independent of - and have no knowledge of - each other. As someone pointed out, the oratab file has a list of databases, but some may have been deleted and the oratab not changed to reflect it.